This invention relates to ladder stiles and ladders produced therefrom.
Timber ladders are the traditional form of ladders used by man since he first descended from trees with the aid of supple vines. These traditional forms of ladders are however subject to a number of drawbacks, the main ones being excessive weight, increasing difficulty of supply of the accepted timbers, the tendencies of the customary timbers to wear on the top and bottom edges of the ladder stiles, and electrical conductivity.
Timber ladders, because of the nature of the materials of which they are constructed, have always been the subject of an excessive weight disadvantage. This weight disadvantage is particularly pronounced in the case of one person portable wooden ladders.
In recent years aluminium and fibreglass composite ladders have been developed to overcome this weight disadvantage. Examples of such ladders are disclosed in FR 2472072, EP 43805, and GB 2065211. Such ladders incorporate their own disadvantages of electrical conductivity and poor weathering capacity respectively.
The weight problem with timber ladders has traditionally been addressed by reducing the cross sectional sizes of the timber members of the ladder and particularly the stiles. An example of such a ladder is provided in U.S. Pat. No. 3734237. However, when it is considered that reducing the depth of the ladder stile has a deleterious effect on the stiffness and strength of that stile according to a logarithmic scale, it will readily be appreciated that a small reduction in the depth of a ladder stile has a larger effect on that stile's stiffness and strength. When it is further considered that the centre of the ladder stile is drilled out to accept the ends of the ladder rungs, it will be readily appreciated that further reductions in the depth of the stile of the ladders is impractical because it becomes dangerous.
Similarly, the thickness of the ladder stiles cannot be further reduced because it is the depth of the housing of the rungs into the stiles which gives the ladder its diagonal bracing, its resistance to twist and its overall structural integrity.
Accordingly, thinner section timber ladders are subject to the drawbacks of being weaker overall structures which only partially solve the excessive weight problem.
Another difficulty associated with the production of timber ladder stiles is that over the years some species of timbers suitable for the manufacture of timber ladder stiles have been felled to extinction in most growing areas and are therefore unavailable for supply to manufacturers and the end user. The timber which is nowadays most used, at least in the Pan Pacific region, for the manufacture of ladders is Douglas Fir, commonly known by its trade name Oregon. Only the virgin forest timber is suitable for ladder stiles, the regrowth, or second crop timber being unsuitable owing to fast growth (weak timber), cross grain and many knots.
This timber is also now becoming difficult to procure owing to the rapidly diminishing resource of virgin forest and also the closure of remaining forests for the protection of local flora and fauna. Moreover, the increasing difficulty of supplying the accepted timbers is a problem which the passage of time will only exacerbate. It has therefore become necessary for manufacturers of timber ladders to find alternatives to these traditional timbers.
The traditional timbers used for the manufacture of timber ladders are soft woods and therefore by their very nature, prone to wear on the top and bottom surfaces of the stiles in situations of moderate or constant use. Also these timbers are not particularly resistant to the wearing effects of sun and rain. These timbers tend to be very stringy, which feature often operates to produce sharp splinters especially at locations of wear or damage. These undesirable features of the traditional timber ladders and especially of the stiles are further constraints to the saleability of such ladders to the public.
Traditional timber ladders also commonly include reinforcing material in the form of a rod running the length of the stile. While this reinforcing material has a moderate effect in increasing the strength and stiffness of the ladder stile, its main purpose is to act as a "safety net" in the unlikely event of a ladder stile breaking into two pieces when in use. Should this happen, the reinforcing material serves to hold the severed pieces together and to prevent the complete collapse of the ladder until such time as the user has been able to descend. Conventionally, a wire reinforcing rod is stapled into a groove on the outside edge of the ladder stile. The disadvantage with this system is that the wire can simply "unzip" by springing all the staples in quick succession.
It is accordingly, an object of the present invention to provide a ladder stile which will go some way towards overcoming the abovementioned disadvantages by providing a lightweight non-conducting ladder stile and ladders formed therefrom, or which at least provides the public with a useful choice.